Drying of freshly harvested paddy rice is essential to insure its conservation. Typically, paddy rice is harvested between 20% and 30% moisture content and good conservation requires less than 14%. Paddy rice is traditionally dried under the sun. This method, although low in cost, has many deficiencies. Sun drying is difficult to control, drying is not even between the top layer and the bottom layer, and rain can ruin exposed grains. Birds, rodents and insects can also substantially damage a harvest. Drying paddy rice in humid climates is especially difficult during the rainy season because the ambient air is already wet, and the rice will not dry unless heated to high temperatures which can damage the quality of the rice as well as reduce the whole grain yield.
The need for better quality finished rice has called for better methods of drying. The International Rice Research Institute (IRRI) and other institutions have studied methods of drying paddy rice based on hot air and given recommendations for optimum conditions of temperature, humidity, flow rates, and time of drying. In short, these conditions can be summarized as follows: lower temperatures are generally better, and the temperature should be kept below 45-50 degrees C.; relative humidity of the drying air should be low enough to insure efficient drying; the air flow rate must be high enough to insure drying throughout the whole mass of the grains; and drying rates of about 2% to 3% moisture reduction per hour are best because faster drying could make the grain brittle.
Until now, the majority of rice dryers relied on the heating of ambient air, with heat sources such as an oil or a coal burner, and the blowing of the hot air into a drying bin containing the paddy rice. This principle is quite appropriate in dry climates, but deficient in humid climates because the ambient air contains high levels of moisture and can only provide marginal drying capacity. Therefore, high temperatures (as high as 60 degrees C.) are used to the detriment of rice quality, and long hours of drying result in high energy costs.
Several types of drying bins are now used. Two of them are quite common in Asia: flat bed dryers and tower dryers. Flat bed dryers are made of a large surface chamber with a perforated floor located about half way up in the chamber, creating a bottom air plenum and a grain storage chamber on top. Hot air is blown into the bottom chamber and admitted into the grain storage chamber through the perforated floor. Although very simple, the flat bed dryers do not provide even drying; the bottom layer dries faster than the top layer. Even drying requires labor intensive and health-hazardous manual turning over, a task which exposes workers to hot, humid air as well as harsh, silicon-rich grain dust.
Another system is the tower dryer, consisting of a tall tower in which grain is fed at the top and allowed to fall to the bottom, while hot air is blown upwards in a counter flow fashion. In a tower dryer, grain does not dry in one pass, but must be recirculated several times, requiring mechanical devices such as a bucket elevator. Because of their complexity, the tower dryers are more expensive and can only be economical in larger capacities. Both of the above dryer systems are very difficult to move, because they are fairly large and require a prepared fixed surface. The flat bed system dries grains in thin layers (typically less than 25 cm) and therefore requires a large surface. The tower dryer, as the name indicates, is usually very tall (on the order of 10 meters) and includes complicated grain conveying devices such as bucket elevators.